The popularity of the gravity model is relatively recent. It was used during the
1960s and 1970s to estimate trade flows but was criticized because it lacks a strong
theoretical foundation. Tinbergen (1962), Poyhonen (1963), and Linneman (1966)
provided initial specifications and estimates of the determinants of trade flows
while Aitken (1973) applied it to RTAs. Anderson (1979) provided a rigorous
economic justification, deriving a reduced-form gravity equation from a general
equilibrium model incorporating the properties of expenditure systems. Bergstrand
(1985) and Deardorff (1997) also provided partial theoretical foundations for the
gravity equation, although none of the models generated exactly the same equation
generally used in empirical work. Due to a revival of interest among economists in
the interconnectedness of economics and geography, the gravity model has again
become popular